The present application claims the benefit of 35 U.S.C. 119(a) of EP Serial No. 98302386.2 filed Mar. 27, 1998, and entitled METHOD AND APPARATUS FOR DRILLING AN OFFSHORE UNDERWATER WELL.
The present invention relates to a method and apparatus for drilling an offshore underwater well.
Two conventional methods exist for drilling an offshore underwater well. The first of these is to drill and set a conductor pipe between a surface platform and the sea bed followed by drilling a surface well using a platform wellhead. The BOP is located on the surface wellhead. Subsequent casing strings are landed in the surface wellhead. The well is completed by suspending completion tubing from the wellhead and installing a platform tree. A second method is to drill and set a conductor pipe into the seabed using a floating drilling vessel with the wellhead located on the bed. A subsea drilling BOP has to run on a drilling riser down to the seabed and is connected to the subsea wellhead. A subsea well is drilled with subsequent casing hangers landed in the subsea wellhead. The well is completed by placing a conventional tree on the seabed wellhead. An alternative subsea option is to use a horizontal tree and then run the tubing.
As the industry moves further offshore and beyond the continental shelf, the water depths being considered are drastically increasing as reservoirs down the flank of the continental shelf and on the ocean floors are discovered. These water depths rule out the use of conventional platforms and their low cost drilling techniques. Floating or tension production platform systems can be used but their drilling footprint into the reservoir is limited, requiring peripheral seabed subsea production support wells. Subsea fields involve considerable complex subsea architecture and require extensive high cost rig intervention.
One way in which an attempt has been made to increase the footprint of a production platform is the provision of a slanted conductor. In such an arrangement, the conductor is supported at an angle by the platform so that it can be run in at an angle thereby increasing the lateral distance between the base of the platform and the location where the conductor meets the seabed. However, such an arrangement is awkward and costly as it requires a specially made structure to support the conductor at an angle. Further, the system will not work in deep water without some support for the conductor at various locations between the surface and the seabed which is not available from a floating platform.
According to the present invention, a method of drilling an offshore underwater well comprises the steps of installing a riser conduit so that it is substantially vertically supported at a production deck situated substantially at the sea surface and deviates progressively further from the vertical with increasing sea depth, fixing the riser conduit at the seabed in a non-vertical orientation, and drilling the well into the seabed at an angle to the vertical.
As the riser conduit is substantially vertically supported at the production deck, it is possible to use conventional platform drilling and production techniques which help keep the costs to a minimum. Further, because the riser conduit is supported at the surface and at the seabed, and deviates progressively further from the vertical in between, intermediate support is not required but can be provided if necessary by buoyancy modules.
In some fields, the reservoir could be relatively close to the seabed. In such a case, there is insufficient depth for a conventional subsea well which starts vertically at the seabed to be deviated to a sufficient angle to access reservoir formations not already being drained by nearby vertical or deviated wells. Therefore only a limited reservoir acreage can be accessed. With the present invention, some of this deviation from the vertical is already provided before reaching the seabed, so that less deviation is required underground which allows higher angle or horizontal wells to be drilled far along the reservoir. This allows better access to reservoirs which are close to the seabed. However, the most important benefit of the present invention arises when the water is sufficiently deep that the riser conduit can be deviated to be horizontal at the seabed. Once the riser conduit becomes horizontal, it is possible to extend it some considerable distance along the seabed before drilling into the seabed so that the drilling footprint of a platform can be greatly increased without drilling.
There are a number of different ways in which the riser conduit can be installed. According to a first method, the riser conduit is run from an installation vessel with a skid attached, installed vertically and pivotally connected at the seabed, the installation vessel is moved horizontally to the production installation while the riser conduit is fed out from the installation vessel, and the riser conduit is transferred to the production installation. According to a second method, the production deck is offset from the location where the riser conduit is connected to a skid and is to be fixed at the seabed, the riser conduit is connected to a skid and is fed down from the production deck and is manoeuvred out to the end target location at the seabed. According to a third method the riser conduit is pre-made and towed to the appropriate location before being fixed at the production deck and fixed at the seabed. In this third case, the pipe may be towed out just off the seabed, and one end raised to the production deck. Alternatively, the pipe may be towed out and hung off at the platform before being lowered to the seabed and fixed.
According to a second aspect of the present invention, an offshore wellhead assembly comprises a production deck at which a riser conduit is vertically suspended, the riser conduit deviating progressively further from the vertical with increasing sea depth, the riser conduit being fixed at an angle to the vertical at the seabed by a fixture, and a cased well extending into the seabed from the fixture. This arrangement provides the same advantages of being able to access reservoirs areas close to the seabed, and increase the drilling footprint of the production installation as referred to above.
The riser conduit may be rigidly locked to the fixture. However, in order to provide ease of installation and a fixture which can accommodate the riser at any angle it is preferable for the riser conduit to be pivotally attached to the fixture.
The fixture is preferably in the form of a skid having a gravity base or piles to secure it to the seabed. The skid is readily able to be transported to the correct location and can be simply secured to the seabed by the base or the piles.